This invention relates in general to polyimide resins and, more specifically, to modified polyimide resins suitable for use as high performance adhesives and as matrix materials for fiber-reinforced composite structures.
Our prior U.S. Pat. Nos. 4,442,283, 4,444,823, 4,183,838 and 4,183,839 disclose and claim certain polyimide compositions which are flame resistant and useful as coatings and adhesives. The coating and adhesive compositions described in the above-mentioned prior patents are made by first preparing a suitable bisimide by reacting an aromatic tetracarboxylic acid dianhydride with a cyclic amide or oxoimine. This bisimide has the typical general formula: ##STR1## where "n" is a positive integer.
The polyimide forming material is then prepared by dissolving the bisimide in an inert solvent; then adding thereto a suitable diamine, producing a viscous fluid containing an intimate, unpolymerized mixture of N-substituted cyclic carboxy terminated bisimide, and diamine which is capable of being converted to a high molecular weight polymer through an exchange reaction by the application of heat.
The following is exemplary of the exchange reaction which occurs: ##STR2## where "n" is a positive integer.
The coating produced is tough, highly adherent to various surfaces, with very few pinholes or bubbles. It has excellent peel strength and is resistant to high temperatures, peeling and abrasion.
These prior polyimide resins, however, are less than ideal for several adhesive applications and for impregnating fibers in the production of composite structures for several reasons. The prior resins tended to require undesirably high curing temperatures. At the higher temperatures the water and alcohol released during polymerization tended to form bubbles or voids in the thicker composite structures. Many of the prior adhesive solutions were monomeric rather than polymeric and therefore released large quantities of water and alcohol.
Thus, there is a continuing need for improved adhesives and matrix materials for fiber-reinforced composites having a better combination of low processing temperatures, lack of alcohols in the final reaction products and greater resistance to the formation of bubbles or voids in composite structures.